Method for manufacturing resonant device

ABSTRACT

A piezoelectric member and an electrode are formed over a silicon substrate. The piezoelectric member and the electrode are patterned by photolithography. The silicon substrate is etched to form a body. A protective film is formed on at least one surface of the body. Another surface having no protective film thereon is etched to obtain a resonant device. The body is etched in its thickness direction accurately while a resonance frequency of the body is measured. The manufacturing processes allow the resonance frequency and a gap frequency of the resonant device to be adjusted to predetermined values.

TECHNICAL FIELD

The present invention relates to a resonant device utilizing mechanical resonance phenomena of substance, for use in, for example, acceleration sensors, angular velocity sensors, resonators, and filters.

BACKGROUND ART

U.S. Pat. No. 5,438,231 discloses an angular velocity sensor, a conventional resonant device includes a body made of non-piezoelectric material, such as silicon, and having a tuning-fork shape, a lower electrode on the body, a piezoelectric member on the lower electrode, and plural upper electrodes on the piezoelectric member. A voltage applied between the lower electrode and the upper electrodes causes the tuning fork-shape body to vibrate. An angular velocity applied to the body from outside during the vibration displaces the body perpendicularly to a vibrating direction due to Coriolis force. The displacement generates a voltage between the upper electrodes and the lower electrode, and this voltage is measured as a displacement, hence allowing the angular velocity to be measured.

The angular velocity sensor including the body having the a tuning fork shape has respective resonance frequencies in a driving direction and a detecting direction of the body. The frequencies are close to each other and separated by a predetermined frequency in order to obtain a high driving efficiency and a high detection sensitivity. The resonance frequencies are determined by the widths and thicknesses of arms of the tuning fork-shaped body. Those dimensions is required to be accurate with small errors thereof not more than ±1 μm.

To obtain the dimensional accuracy, the body is processed in its width direction by photolithography and etching, and is processed in its thickness direction by machining. Those processes are important factors to determine absolute values of respective resonance frequencies in the width direction and the thickness direction of the body having the tuning fork-shape. Not only the absolute values of the resonance frequencies but also a difference between the resonance frequencies, i.e., a gap frequency needs to be determined to be a predetermined value.

Indexes indicating that they are the predetermined values are only the shapes and dimensions of the arms, namely, the widths and the thicknesses of the arms while the body is processed. Therefore, it is difficult to measure actual resonance frequencies exactly. That is, it is difficult to use the gap frequency as an index for processing the body by the foregoing method.

SUMMARY OF THE INVENTION

A piezoelectric member and an electrode are formed over a silicon substrate. The piezoelectric member and the electrode are patterned by photolithography. The silicon substrate is etched to form a body. A protective film is formed on at least one surface of the body. Another surface having no protective film thereon is etched to obtain a resonant device. The body is etched in its thickness direction accurately while a resonance frequency of the body is measured. The manufacturing processes allow the resonance frequency and a gap frequency of the resonant device to be adjusted to predetermined values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an angular velocity sensor, a resonant device in accordance with Exemplary Embodiments 1 to 3 of the present invention.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1, for illustrating processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 3 is a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 4 shows a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 5 is a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 6 is a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 7 is a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 8 is a sectional view taken along line 2-2 of FIG. 1, for illustrating the processes of manufacturing the angular velocity sensor of Embodiment 1.

FIG. 9 is a perspective view of the angular velocity sensor mounted in a package in accordance with Embodiment 1.

FIG. 10 is a sectional view taken along line 10-10 of FIG. 9, for illustrating processes of manufacturing the angular velocity sensor shown in FIG. 9.

FIG. 11 is a sectional view taken along line 10-10 of FIG. 9, for illustrating the processes of manufacturing the angular velocity sensor shown in FIG. 9.

FIG. 12 is a sectional view taken along line 10-10 of FIG. 9, for illustrating the processes of manufacturing the angular velocity sensor shown in FIG. 9.

FIG. 13 is a sectional view taken along line 2-2 of FIG. 1, for illustrating processes of manufacturing another angular velocity sensor in of Embodiment 1.

FIG. 14 is a sectional view taken along line 2-2 of FIG. 1, for illustrating processes of manufacturing an angular velocity sensor of Embodiment 2.

FIG. 15 is a sectional view taken along line 2-2 of FIG. 1, for illustrating processes of manufacturing an angular velocity sensor of Embodiment 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary Embodiment 1

FIG. 1 is a perspective view of an angular velocity sensor, a resonant device in accordance with Exemplary Embodiments of the present invention. The sensor includes body 1 having a tuning-fork shape and made of silicon, lower electrode 2 on an upper surface of body 1 and made of platinum, driving piezoelectric member 3A and detecting piezoelectric member 3B on an upper surface of lower electrode 2 and made of lead zirconate titanate, driving electrode 4A on an upper surface of driving piezoelectric member 3A, and detecting electrode 4B on an upper surface of detecting piezoelectric member 3B. Lower electrode 2 and upper electrode 4 are connected to the outside via lead-wires and electrode pad 6 on base 7 of body 1.

An operation of the angular velocity sensor will be described below with reference to FIG. 1. Upper electrode 4 is separated into driving electrode 4A and detecting electrode 4B. Driving piezoelectric member 3A is located between electrode 4A and lower electrode 2, and detecting piezoelectric member 3B is located between the electrode 4 b and lower electrode 2. A voltage applied between driving electrode 4A and lower electrode 2 causes driving piezoelectric member 3A to expand and contract, thereby distorting arms 9A and 9B of body 1 having the tuning-fork shape. Thus, arms 9A and 9B vibrate in direction 51 parallel to a plane including the arms.

Rotational motion about rotation axis 50 parallel to arms 9A and 9B occurs while arms 9A and 9B vibrate, thus generating a Coriolis force to deflect arms 9A and 9B in direction 52 perpendicular to vibrating direction 51. The deflecting of the arms generates electric charges at detecting piezoelectric member 3B, and detecting electrode 4B detects the charges. In other words, the angular velocity sensor utilizing piezoelectric phenomenon and reverse voltage phenomenon of piezoelectric member 3 for driving arms 9A and 9B as well as detecting an angular velocity. Therefore, large displacements of arms 9A and 9B allow the angular velocity to be detected easily at a high sensitivity.

More specifically, a large amount of charges proportionate to the amount of the distortion of piezoelectric member 3A can be detected. This allows a driving frequency to be set close to a resonance frequency in a driving direction of body 1 having the tuning-fork shape. Then, an amount of the displacement during the driving increases, thus increases a displacement due to the angular velocity. As a result, the angular velocity can be detected easily. In order to obtain a large displacement of piezoelectric member 3B caused by the angular velocity during the rotation, both the resonance frequency in direction 51 for driving the arms and the resonance frequency in direction 52 for deflecting the arms are preferably close to each other. If the frequencies are apart from each other, the displacement of the arms due to the angular velocity hardly transmit in the detecting direction.

However, if the resonance frequencies are too close to each other or identical to each other, respective vibrations in directions 51 and 52 are coupled. This coupling causes the displacement due to the angular velocity is hidden behind the resonance, hence disabling the displacement to be detected. In other words, the resonance frequencies need to be as close as possible while the vibrations are not coupled. For instance, if the resonance frequency in the driving direction is 20 kHz, the resonance frequency in the detecting direction is desirably about 19.8 kHz. A gap frequency, i.e., the difference between the frequencies is preferably not more than about 1% of the resonance frequencies. Meanwhile, the vibrations may be coupled due to shapes of arms 9A and 9B of body 1. The coupling may occur particularly if arms 9A and 9B have sections shaped in parallelogram, as shown in FIG. 13.

A method of manufacturing the angular velocity sensor will be described below with reference to FIGS. 2 to 12. FIGS. 2 to 8 are sectional views taken along line 2-2 of FIG. 1, for illustrating processes of manufacturing the angular velocity sensor of Embodiment 1. FIG. 9 is a perspective view of the angular velocity sensor mounted in a package according to Embodiment 1. FIGS. 10 to 12 are sectional views taken along line 10-10 of FIG. 9, for illustrating processes of manufacturing the angular velocity sensor shown in FIG. 9.

As shown in FIG. 2, platinum layer 102 to form lower electrode 2 is formed on silicon substrate 14 by sputtering. Then, lead-zirconate-titante layer 103 to form piezoelectric member 3 is formed on platinum layer 102 by sputtering, sol-gel process, or a hydrothermal synthesis method. Gold layer 104 to form upper electrode 4 is formed on lead-zirconate-titante layer 103 by an ordinary method for forming a thin film, such as sputtering method or vacuum deposition method.

Next, as shown in FIG. 3, a pattern of resist mask 12 on gold layer 104 is formed by photolithography. Then, as shown in FIG. 4, gold layer 104 and lead-zirconate-titante layer 103 is etched to form electrode layers 4A and 4B and piezoelectric layer 3A and 3B. In order for upper electrode 4 and piezoelectric member 3 to be etched simultaneously with the same mask, the etching preferably causes little side-etch, hence employing dry-etching in this process. Upper electrode 4 is separated into driving electrode 4A and detecting electrode 4B, and piezoelectric member 3 is separated into driving piezoelectric member 3A and detecting piezoelectric member 3B in this process.

Next, as shown in FIG. 5, resist mask 13 is provided on electrodes 4A and 4B and portions of platinum layer 102, and then, platinum layer 102 is etched, thereby providing lower electrode 2. Both dry-etching and wet-etching can be used in this process. If being dry-etched, platinum layer 102 is etched by an ordinary reactive-ion etching using CF₄ gas or Argon gas. If being wet-etched, platinum layer 102 is etched with mixture solution of hydrochloric and nitric acid.

Then, as shown in FIG. 6, resist mask 13 is used consequently to dry-etch silicon substrate 14 with etching gas including first gas for facilitating the etching and second gas for suppressing the etching. The first gas, which may be SF₆ or CF₄, facilitates the etching not only in a depth direction but also in a lateral direction. Thus, the second gas, such as CHF₃ or C4F8, for suppressing the etching is mixed with the first gas forms protective film 5 made of carbon fluoride polymer on an etched lateral surface. The dry-etching therefore can proceed only downward. As a result, as shown in FIG. 7, silicon substrate 14 can be etched downward only. In order for the etched lateral surface to be perpendicular to the surface of substrate 14, substrate 14 is etched a little with the facilitating gas, and protective film 5 is formed a little with the suppressing gas. Then, these processes are repeated alternately, thus allowing the lateral surface to be perpendicular to the surface of substrate 14. More specifically, 130 sccm of SF₆ is supplied for generating plasma for 7 seconds, thereby etching substrate 14, then protective film 5 having a thickness of 0.01 μm is formed. These processes are repeated alternately about 180 times to form the lateral surface along a depth of 200 μm, thus providing the body 1 having the tuning-fork shape.

A magnitude to be etched of carbon fluoride polymer varies according to a type of etching gas more than silicon. Thus, an appropriate selection of the etching gas can etch only the silicon.

Protective film 5 is formed not only on the lateral surfaces of arms 9A and 9B of body 1 but also on the under surface of substrate 14 with the suppressing gas during the etching process. However, protective film 5 formed on the under surface can be removed with the facilitating gas more easily than that on the lateral surfaces, thus allowing the etching to proceed only downward. The etching continues until the under surface of silicon substrate 14 is etched, thus enabling body 1 having the tuning-fork shape to be cut from substrate 14. The etching preferably terminates when the substrate is etched with plasma generated by the suppressing gas. This ensures to form protective film 5 made of carbon fluoride polymer on the lateral surfaces.

Then, as shown in FIG. 8, resist film 13 is removed. Protective film 5 formed on resist film 13 is also removed simultaneously, while film 5 on the lateral surfaces of body 1 made of silicon substrate is not removed.

Next, as shown in FIG. 9, body 1 having the tuning-fork shape is mounted in a packaging member, such as package 8. Then, an electrode coupled to outer electrode 11 at the outside of package 8 is connected to electrode pad 6 on base 7 via wire 10. The package member is not limited to package 8, and may be a member having another shape, such as a board having a plate shape.

As shown in FIG. 10, arms 9A and 9B of body 1, which is mounted in package 8, float in package 8. As shown in FIG. 11, while body 1 mounted in package 8 is pit into etching apparatus 15, the resonance frequency in detecting direction 52 deflecting by Coriolis force is measured with measuring device 16. Then, as shown in FIG. 12, xenon-fluoride (XeF₂) gas is introduced from gas-inlet 17, so that only the under surface of body 1 where silicon is exposed is etched. This etching lowers gradually the resonance frequency in detecting direction 52. The resonance frequency is measured and fed back to the etching, thus allowing the gap frequency, i.e. the difference between the resonance frequency and the driving frequency to be equal to a predetermined value. When the resonance frequency reaches to the predetermined value, XeF₂ gas stops.

Since the lateral surfaces of body 1 having the tuning-fork shape is protected by protective film 5, the lateral surfaces are not etched, and the resonance frequency of the vibration due to the driving in the driving direction does not change. The upper surface of body 1 is covered with lower electrode 2, piezoelectric member 3, and upper electrode 4, so that the upper surface cannot be etched.

Exemplary Embodiment 2

FIG. 14 is a sectional view of a body of an angular velocity sensor in accordance with Exemplary Embodiment 2 of the present invention. The sensor, differently from that of Embodiment 1, a protective film made of carbon-fluoride polymer is formed on an under surface of the body instead of a lateral surface before an etching with xenon fluoride gas, as shown in FIG. 14. The protective film causes the xenon fluoride gas to etch exclusively silicon of the lateral surface of the body. As a result, only a resonance frequency in a driving direction decreases according to progress of the etching. This allows a gap frequency between resonant frequencies of the driving direction and a detecting direction to be adjusted.

Exemplary Embodiment 3

FIG. 15 is a sectional view of a body of an angular velocity sensor in accordance with Exemplary Embodiment 3 of the present invention. The sensor, differently from those of Embodiments 1 and 2, a protective film made of carbon fluoride polymer is formed not only on a lateral surface or an under surface of the body, but also on portions of both the lateral surface and the under surface. Portions of corners of the body are exposed, as shown in FIG. 15. In FIG. 15, a body may have a cross-section having a parallelogram shape, and have corners having acute-angled sections exposed.

This structure allows xenon fluoride gas to etch the exposed corners exclusively. In the body having the cross section having the parallelogram shape, respective vibrations in a driving direction and a detecting direction may be coupled with each other, however, the foregoing structure can cancel the coupling.

In the method of manufacturing the resonant device according to the embodiments of the present invention, the body is etched while the resonance frequency in the detecting direction is measured, thereby having the resonance frequency to be adjusted to a predetermined value accurately. Further, the etching with the xenon-fluoride gas does not require generating plasma, thus allowing the resonance frequency to be accurately measured without influence of a noise caused by the plasma.

INDUSTRIAL APPLICABILITY

In a method of manufacturing a resonant device, the device is etched accurately in its thickness direction while its resonance frequency is measured, thus having a gap frequency of the resonant device to be adjusted to a predetermined value. 

1. A method of manufacturing a resonant device, said method comprising: forming a piezoelectric layer and an electrode layer over a surface of a silicon substrate; forming a body by etching the silicon substrate, wherein the body has a first surface, a second surface, and a third surface corresponding to the surface of the silicon substrate, the second surface is parallel to the third surface, and the first surface is non-parallel to the third surface; after said forming of the body, forming a protective film on a first surface of the body crossing the upper surface of the silicon substrate; and after said forming of the protective film, etching a second surface of the body parallel to the upper surface of the silicon substrate.
 2. The method of claim 1, wherein said forming of the body comprises etching the silicon substrate with first gas for facilitating etching and second gas for suppressing etching.
 3. The method of claim 2, wherein said forming of the body comprises etching the silicon substrate with mixture gas of the first gas and the second gas.
 4. The method of claim 2, wherein the first gas comprises SF₆, and the second gas comprises one of C₄F₈ and CHF₃.
 5. The method of claim 1, wherein the protective film comprises carbon fluoride polymer.
 6. The method of claim 1, wherein said etching of the second surface of the body comprises dry-etching the second surface with XeF₂ gas.
 7. The method of claim 1, further comprising measuring a resonance frequency of the body during said etching of the second surface of the body, wherein said etching of the second surface of the body comprises controlling an amount of the etched second surface of the body according to the measured resonance frequency measured.
 8. The method of claim 7, wherein said controlling the amount of the etched second surface comprises stopping said etching of the second surface of the body when the resonance frequency becomes equal to a predetermined value.
 9. The method of claim 7, further comprising mounting the body to a packaging member, wherein said measuring of the resonance frequency of the body comprises measuring the resonance frequency of the body while the body is mounted to the packaging member.
 10. The method of claim 1, wherein the body has a tuning-fork shape.
 11. A method of manufacturing a resonant device, said method comprising: forming a piezoelectric layer and an electrode layer over a surface of a silicon substrate; forming a body by etching the silicon substrate; etching the body; measuring a resonance frequency of the body while performing said etching of the body; and controlling an amount of the etched body according to the measured resonance frequency measured while performing said etching of the body.
 12. The method of claim 11, wherein said forming of the body comprises etching the silicon substrate with first gas for facilitating etching and second gas for suppressing etching.
 13. The method of claim 11, wherein said forming of the body comprises etching the silicon substrate with mixture gas of the first gas and the second gas.
 14. The method of claim 12, wherein the first gas comprises SF₆, and the second gas comprises one of C₄F₈ and CHF₃.
 15. The method of claim 11, wherein said etching of the body comprises dry-etching the body with XeF₂ gas.
 16. The method of claim 11, wherein said controlling the amount of the etched body comprises stopping said etching of the body when the resonance frequency becomes equal to a predetermined value.
 17. The method of claim 11, further comprising mounting the body to a packaging member, wherein said measuring of the resonance frequency of the body comprises measuring the resonance frequency of the body while the body is mounted to the packaging member.
 18. The method of claim 11, wherein the body has a tuning-fork shape.
 19. The method of claim 11, wherein said etching of the body comprises etching a first surface of the body.
 20. The method of claim 11, wherein said etching of the body comprises etching a corner of the body. 